Conventionally, the position with respect to the thrust direction of the shaft of a small-sized motor equipped with a worm gear reducer is fixed, for example, as shown in FIG. 8 (Japanese Patent Publication (kokoku) No. 60-11536/1985). Reference numeral 1 denotes a motor body; reference numeral 6 denotes a worm coupled with a motor shaft 4; and reference numeral 7 denotes a worm wheel engaged with the worm 6. As shown in FIG. 8, an adjustment screw 43 is attached to a gear case 17 at a position corresponding to a front end portion of the motor shaft 4. After thrust is adjusted by means of the adjustment screw 43, the adjustment screw 43 is fixed by means of a nut 44. In thrust adjustment, the motor is rotated, while a specially machined front end of the adjustment screw 43 is brought in contact with the end of the motor shaft 4. While a no-load current of the motor is being observed, the adjustment screw 43 is positioned where the no-load current assumes a value as small as possible and the motor shaft 4 has no play in the thrust direction. The adjustment screw 43 is fixed in the position by means of tightening the nut 44. Such a manner of fixation of the motor shaft 4 with respect to the thrust direction requires skill in fixing the adjustment screw 43 and the nut 44 together, thus consuming much time.
Japanese Patent Publication (kokoku) No. 60-11536/1985 also discloses a technique for positioning with respect to the thrust direction by use of a spacer to be heated through electromagnetic induction. In addition to a shaft insertion hole, a square hole is provided. The spacer is inserted through the square hole and toward the front end portion of the shaft. The front end portion of the shaft is brought into direct contact with the spacer and is heated through electromagnetic induction. The front end portion of the shaft is then pressed against the softened spacer to thereby be positioned with respect to the thrust direction.
Since this positioning mechanism requires formation of the square hole in addition to the shaft insertion hole, an associated gap may allow entry of water from the outside. Also, frictional heat generated through shaft rotation during motor operation or heat generated from windings and a commutator during motor operation is transmitted to the spacer through the shaft, potentially re-softening the spacer. Further, since the spacer by itself bears a radial force and a thrust force, wear or breakage of the spacer is likely to cresent a problem.
Other conventional techniques for fixing a motor shaft with respect to the thrust direction include the following. As shown in FIG. 9, resin 46 is injected into a cavity portion located between a gear case 17 and a motor shaft 4 through an injection inlet 45 extending between the cavity portion and the exterior of the gear case 17, thereby fixing the position of the motor shaft 4 with respect to the thrust direction (Japanese Patent Publication (kokoku) No. 61-56701/1986). Similarly, as shown in FIG. 10, the resin 46 is injected into a cavity portion located between the gear case 17 and a bearing holder 22 equipped with a bearing device (radial bearing 5 and steel ball 9) for the motor shaft 4, through the injection inlet 45 extending between the cavity portion and the exterior of the gear case 17, thereby fixing the position of the motor shaft 4 with respect to the thrust direction (Japanese Patent Publication (kokoku) No. 7-106044/1995).
Such resin-injection practice requires labor and time for curing of the applied resin. Also, deformation, such as shrinkage, or time-course change of the resin causes a decrease in adhesion between the resin and the gear case with a resultant formation of a gap between the resin and the gear case at an injection inlet portion. Water may enter the gear case through the gap from the outside.
A certain kind of resin may require control of injection pressure or injection temperature. In the case of a resin that requires mixture and reaction of two components before use, a mixing process requires labor, and resin control must be performed before and after the mixing process. In addition, at the time of resin injection, the amount of resin to be injected must be controlled or adjusted, and care must be taken in order to prevent mixing of bubbles into the resin. In a configuration where resin to be used directly supports a motor shaft, a problem may arise due to wear or breakage of the resin caused by a thrust of the motor shaft.